This invention relates to an electric fuse, and more particularly, to a high voltage current-limiting fuse that is capable of interrupting a wide range of currents and is especially suited for low current interruption.
The usual high voltage current-limiting fuse comprises at least one fusible conductive element connected in series with the circuit being protected. When a overcurrent flows through the fusible element for a predetermined duration, the fusible element melts at one or more restricted locations along its length, establishing an arc in each region where melting occurs. If such a fuse is operated by a low current, such as 1.5 times its continuous current rating, only a single arc might be created in response to the overcurrent condition.
The formation of only a single arc presents problems for a high voltage fuse. For example, for a fuse to successfully interrupt 15 kV using a single arc, the arc length must be rapidly increased to a relatively great value in the range of 25.4 to 76.2 cm (10 to 30 inches). Moreover, this relatively long single arc must be developed within a few cycles of power frequency current, or the electric field in the arc will diminish to an unacceptably low value, and the fuse will fail to clear. Developing such a long arc within the required time is not usually feasible, considering the slowness with which the arc will elongate when the current density is low. Accordingly, it is desired that more than one arc be created along a fuse element in response to low overcurrents producing operation of a high voltage fuse at voltages above about 1 kV.
Various means are known to establish multiple breaks for a high voltage fuse element in order to facilitate clearing for low current fault interruption. One such means is taught in my U.S. Pat. No. 4,357,588, assigned to the same assignee of the present invention, and herein incorporated by reference.
U.S. Pat. No. 4,357,588 describes fuse elements including various reduced cross-section portions having a desired fusible time-current characteristic which causes rupturing of the fuse elements and which fuse elements are especially suited for low current fault interruption. Although the reduced cross-sectional portions of the fuse elements provide for the desired low current interruption, the operation of these fuse elements is hindered inasmuch as there is a minimum current density in the reduced cross-section portions below which multiple melting will not occur. This current density corresponds to a melting time of 1-2 hours.
There is a requirement for a fuse to be capable of clearing currents which cause melting in times longer than 1-2 hours, and indeed it is desirable that a fuse be capable of clearing any current which causes its element(s) to open. This should include cases where the fuse elements have been damaged, for example, by a large surge current, and the fuse actually opens when carrying less than its rated current. It is toward this end that the present invention is directed.
Another approach for achieving multiple breaks in response to persistent overcurrents of low value is disclosed in U.S. Pat. No. 3,705,373--Cameron. Cameron provides a main fusible conductive element and an auxiliary conductive element electrically connected to the main element at at least two spaced points along its length. The auxiliary element is made entirely or at least partially of high-resistivity exothermic material so that current normally flows through the main fusible element. If, in response to an overcurrent, the main fusible element melts at a location between said two points, current is diverted into the auxiliary element, causing the material of the auxiliary element to exothermically react. Since the auxiliary element is closely adjacent or touching the main fusible element, the exothermic reaction heats the main fusible element and causes it to melt at one or more locations in addition to the first location.
This fuse has a number of significant disadvantages. One is that the exothermic material must be conductive to allow it to be formed as a conductive element, and this limits the type and quantity of the exothermic material that can be selected for such use. Another disadvantage is that a relatively large quantity of exothermic material is needed to effect melting of the relatively large fusible element present in a high current fuse; and the presence of this large quantity of conductive exothermic material results in an undesirable parallel conductive path close to the main fusible element after fuse operation, and this would be detrimental to final clearing of the fuse. Still another disadvantage is that in the case of a fuse with multiple main fusible elements in parallel, a plurality of auxiliary elements of exothermic material, one for each main fusible element, would be needed. Still another disadvantage is that the auxiliary element cannot respond to all breaks in the main fusible element. For example, should a break occur in the main fusible element only in a location outside the region spanned by the auxiliary element, the auxiliary element would fail to respond since it would still be shunted by an intact portion of the low resistance main fusible element. Still another disadvantage of the Cameron design is that the auxiliary element must be closely adjacent the main element in order to effect a consistent response of the main element following the exothermic reaction.